Well, it's many problems, that's the fun of the example. But yes, I think left join is fundamentally about grouping, and grouping would be better solved with s

Well, it's many problems, that's the fun of the example. But yes, I think left join is fundamentally about grouping, and grouping would be better solved with set-valued attributes or relation-valued attributes. I wouldn't call that "hierarchically-structured data" because that has the connotation that the data is actually stored in a hierarchy, which I think is generally a bad idea (though not always).